It's not even a "what if". And besides, there's a saying in the railroad: the safest train is a stopped train. In other words, as soon as you start moving a train, you increase the level of danger, no matter what you do. That's a tradeoff we make as a society. And that tradeoff is much higher in cars than in trains (hence, why public transit can legitimately be seen as a lifesaving measure).

How much a driver can do if a train breaks down depends on the train and how much the driver knows about his engine. Unfortunately, with a lot of the engines being computer controlled, the amount a driver can do has reduced - but there are still plenty of things a driver can repair. And sometimes it's just a matter of bypassing problems. For example, there's a problem on the track and you need to give the driver explicit instructions on how to proceed and what to watch for. That's not something you can easily tell a self driving train to do - that train will most likely be stuck until someone remotes in. Assuming, of course, that the remote connection isn't shot. Then you're absolutely screwed.

And even if the train breaks down completely, it's still important to have someone there so that the passengers aren't left alone. Who can help evacuate the train if necessary. Who can communicate to the passengers and be there directly if there are specific problems that need to be addressed.